Such a method is already known in the art, e.g. from the article `An ATM switching architecture with intrinsic multicast capabilities for the Belgian broadband experiment`, by M. De Prycker et.al., Proceedings ISS 1990, Stockholm, May 1990, Vol V, pp. 111 to 118. Therein, the switching network is part of a local exchange via which connections may be made between e.g. an audio/TV distribution centre and subscriber stations. The switching nodes are referred to as switching elements and operate according to the Asynchronous Transfer Mode (ATM) protocol. The cell stream routed from the above one network inlet to the first set of network outlets, i.e. to each network outlet of this first set, is thus routed along a cluster of branches each leading via various ones of the above switching elements, such a cluster being referred to as a multicast tree in the above article and hereinafter. The modification of the first set of network outlets consists in the addition of network outlets to obtain the second set, i.e. in the addition of branches to the existing multicast tree. Thereto, the latter switching network being a so-called connection oriented switching network, so-called set-up cells are launched along this tree using the branches thereof as far as possible. Once the set-up cell can no longer use such branches in a switching node and therefore has to leave the multicast tree, the required resources are allocated in this switching node and a new branch is added to the existing multicast tree, thereby modifying the first set into a second set.
However, in a so-called connectionless switching network the latter way of modifying a multicast tree may not be applied since no connection is set up therein with a set-up cell but cells are routed therein to their respective destination which is indicated by a routing tag. In that case, a central controller may be provided which controls all switching nodes routing cells of the cell stream to their respective destination, the addition of a network outlet thus requiring extensive processing in this central controller. Indeed, this central controller must control all the latter routing switching nodes and must communicate thereto the information necessary for modifying the multicast tree.